Deck cranes of the derrick type are commonly employed on board ship and on oil rigs. Such deck cranes normally comprise a main structure mounted on a short fixed post which rotates on top of the post, the structure carrying pivoted thereto a jib and luffing and hoisting gear connected between the structure and the jib. The connection of the structure to the post comprises a thrust bearing usually consisting of outer and inner ring members, the inner ring member being fixed to the post and the outer ring member being connected to the structure and carrying gear teeth engaged by a driving pinion.
In such a known construction the thrust bearing has to carry almost all the static and live loads imposed by the jib. These are the very eccentric loads comprising the weight of the jib itself and the weight of any load supported by the jib, also the slewing forces, luffing forces and load acceleration and deceleration forces. The bearing ring is thus under continual forces tending to twist it on the post. In addition to such twisting load the structure presents a considerable area to the wind and a high wind blowing against the crane either augments the twisting force on the bearing ring or introduces a fresh twisting force in another direction further complicating the stress conditions. These troubles can be even further augmented if it should happen that the crane hook becomes snagged on a portion of the ship's structure or the structure of an oil rig while the crane is hoisting. This can happen unknown to the crane operator so that an unbalance situation too severe to be corrected can arise before the operator can take action to retrieve the situation. The bearing is actually the weakest part of the crane structure and during the situations described above the twisting on the bearing can be such as to cause the movable portion of the bearing supporting the jib and all the rest of the structure of the crane apart from the mast to twist off the mast so that the crane structure apart from the mast topples even falling overboard from a ship or an oil rig. There have already been several fatal accidents because of the occurrence of these conditions.
To overcome this disability derrick cranes have heretofore been constructed with the jib mounted on a long sleeve rotatable around the mast on two sets of spaced roller bearings, one set being arranged to resist axial thrust and the other set being arranged to resist radial thrust.
This construction is complicated, expensive and clumsy since it requires either that the sleeve should extend below upper deck level, in which construction the most convenient method of slewing is by means of a cable wound around the sleeve and connected to a winch, although here there is a limitation imposed on the available angle of slew and this construction requires a large hole to be cut in the upper deck for passage of the rotatable sleeve, or that the sleeve should extend upwardly above upper deck level adding to the deadweight trying to capsize the ship. Also two sets of spaced roller bearings often give rise to trouble because of problems of alignment resulting from wear and temporary distortion of the structure particularly when the derrick is lifting heavy loads and starting and stopping heavy loads.